Copper pyrometallurgy can be summarized as a multi-step process wherein each step is responsible for removing one or more impurities to ultimately obtain pure copper metal. These impurities include, in addition to non metallic compounds, a variety of metals present in the feeding material of the primary pyrometallurgical vessel of the smelter. Some of these metals, like silver and gold, are desirable because of their value and they will find there way in the copper produced from the smelting operation. They will then be extracted during subsequent copper refining. Therefore, necessary efforts are made to ensure that these precious metals are not lost during the pyrometallurgical process.
During the smelting of copper concentrates and matte converting, metals like lead, mercury, arsenic, tin, cadmium, bismuth, silver, zinc, etc. are completely or partially volatilized, and therefore end up in the gases exiting the furnace. The gas stream will also carry small splash particles from the slag and the matte. Likewise, these particles contain precious metals like silver and gold, as well as copper. In the gas cleaning operations, most of the volatilized metals will be condensed and recovered as dust in electrostatic precipitators. The particle size of this fine dust is on the order of 2 microns or less, contains mainly sulphates, like lead, copper and zinc sulphates, and oxides, like tin and bismuth oxides. Chlorides like silver chloride may also be present in smaller concentrations. The splash particles are on the other hand bigger, typically from a few tens to a few hundreds of microns, and also recovered in the precipitators. The flue dusts therefore comprise a fine dust mixed with coarser particles.
Although flue dusts are an effective bleed for removing undesirable impurities from copper matte, they nevertheless contain non-negligible amounts of precious metals, which means that their disposal represents a significant economic loss, and may also create unacceptable environmental problems. For these reasons, most industrial copper smelters recycle most of their flue dusts in the primary pyrometallurgical vessel, particularly that recovered from the precipitators. The unrecycled fraction of the flue dusts is generally forwarded directly to a lead smelter, and may sometimes be treated prior to being forwarded there. This operation constitutes important costs for copper smelters. The treatment of lead sulphate is indeed expensive because heavy fines are imposed by lead refiners if impurities like arsenic, bismuth, cadmium, copper, tellurium, chlorine etc. are present in the dusts used as feed materials for lead smelters.
Flue dusts can be treated pyrometallurgically, such as for example in U.S. Pat. No. 4,013,456 or U.S. Pat. No. 5,256,186, but generally, hydrometallurgical methods are much preferred. It is well known that metals like copper, zinc, cadmium and arsenic are already soluble in water at various degrees. U.S. Pat. No. 4,891,067 proposes the selective solubilization of lead, arsenic, molybdenum and antimony in a caustic media.
The use of sulphuric acid for increasing the dissolution of copper and other impurities is conventionally done by many smelters throughout the world, and several publications acknowledge this fact. See for example: Piret in the 3.sup.rd International Symposium on Recycling of Metals and Engineered Materials, 1995, 189-214; Minoura et al. in Metallurgical Review of the MMIJ, 1984, 1 (2), 138-156;Shibasaki et al. in Hydrometallurgy, 1992, 30, 45-57; and Ino et al. in Proceedings of Copper 95 International Conference, 1995, 3, 617-627. Under higher sulphuric acid concentrations, a significant amount of bismuth is solubilized, which may be recovered by dilution, cementation or neutralization, as illustrated in Hanks et al. in Transactions of IMM, Section C, 1979, 99-106.
The addition of chloride ions, either through sodium chloride or hydrochloric acid, solubilizes bismuth as well as a significant amount of silver present in flue dusts. See Asahina et al. in Proceedings of the joint Meeting MMIJ-AIME, 1977, 2, 856-874; and Hydrometallurgy, 1985, 14, 93-103. U.S. Pat. No. 5,443,622 is concerned with a process using a liquor containing chlorides and fluorides to solubilize bismuth and the recovery of cadmium, arsenic and bismuth.
In all of the above methods and processes, the lead present remains in the form of impure lead sulphate that must be either recycled to the primary pyrometallurgical vessel of the copper smelter or forwarded to a lead smelter with all the economic drawbacks described above.
In the treatment of sulphur ores containing lead, as for example in U.S. Pat. No. 4,266,972 and U.S. Pat. No. 4,410,496, lead and most other metals in the ore are leached under oxygen overpressure in an acidic media containing chloride ions. Subsequent metals recovery in the pregnant solution is realized by various known techniques.
U.S. Pat. No. 4,372,782 discloses a method for recovering lead and silver from ores and side products containing lead sulphate. In this method, lead is solubilized in an acidic solution having a pH of about 1,5 and highly concentrated in chloride ions, e.g., 269 g/L of NaCl. Calcium chloride is used to eliminate the excess of sulphate and to increase the solubility of the lead up to about 18 g/L. Following a liquid/solid separation, lead oxychloride is precipitated by adding lime. Subsequently, the oxychloride is calcined at 400.degree. C. with lime to produce calcium plumbate which will be forwarded to a lead smelter. This process does not allow the elimination of bismuth from the lead, and because of the low solubility of lead sulphate in the media, i.e., 18 g/L, a significant capital investment is required to be able to treat tens of tons of flue dusts rich in lead that are produced daily in conventional copper smelters.
It would therefore be highly desirable to develop an efficient process for the hydrometallurgical treatment of flue dusts produced from smelters. Such process should allow a substantially complete extraction and recovery of lead and precious metals economically. Moreover, the lead or lead derivatives obtained should be of good purity to eliminate fines charged by lead smelters for refining such product.